AMT Jfet with triode limitation

[demym]

Hi guys,

i've read with much interest the following article by AMT:

http://www.amtelectronics.com/support/articles/why_tube-like_cannot_measure_up_to_tube_inside/


I also tried to implement the schematic at the bottom of the page:




To my ear, a stage using the above configuration has effectively a more 'soft' response (i tried used the one and only germanium diode i had at home). Also some Schottky diode could be used (but have never dealed with one).

Unfortunately, the two resistor values proportion is not indicate (r and R on the source). I supposed that r would be half of R (i mean, if the stage had originally a 1k resistor, the mod would require 2 470 resistors).

Also, i suppose Rg and Rh are not significant in the schematic (as they are in another color and even represented by a spiced line)

I read with attention the article, but no reference to R and r values (or to their dividing factor); anyone has tried this kind of pseudo triode trick ? Does r and R have a particular meaning (referring to their values) in electronics drawing ?

The author also says that this trick represents the basis for their Legend Amp Series (and samples of the various pedals, M1, B1, E1, F1, R1, ecc.., on youtube are very good sounding !!)

I am trying to implement the trick on some of my pedals, just wanted to know how to read those resistor values.


Thanks in advance !

[GibsonGM]

I know this is not much, but try to find Rtotal for the source, then break it in half....if that doesn't work, find R, and make r MUCH larger than R....you may have to do some experimenting!! 

[demym]

Hi GibsonGM,

so i've tried what you suggested (using a Schottky diode, this time... its effect is more evident than the germanium diode i used before);

As my original stage has 1k on the source (Rtotal), i used 1k as r (the above portion in series with the schottky), and 68ohm as the bottom R.... i this way the stage gets a 'color' that's very pleasing.. it also distorts some more, but i feel a more 'softness' sensation in the clipping. The thing, instead, doesn't work using 1k as R and 68ohm as r (as i thought it should have been).. If there's an effect with this second combination, well, i can't hear it...

Need to try this on some of my jfet emulators (substituting the source resistors with this new 'discovery')..



Also, there's an alternate methode of limiting, using a dc line to feed (via a resistor) the gates of the fets... I've tried one a SLO schematic, and that richens the sound, in my opinion... The schematic i'm talking about is this:



I've tried to hear the difference from those 1.5V lines, connecting them as usual to ground, and i think there's a big difference... without the 1.5V feeding, the sound is as 'dead', and gets richness reconnecting the 1.5v points to that voltage.. I used a trimmer instead of R19 and R20, and i get my optimum at 1.3v... it seems to be some sort of sweet spot tweaking in there.

Anyone else tried these ? I'm struggling to understand those russian forums.. but automatic translations are not adequate...


The AMT Legend amps, anyway, sounds killer on YT videos !!

[Zapp Brannigan]

Demym, this schemo needs very careful tuning with a scope and a AF-generator. On the author's page there is a description of tuning procedure: you need to set 4.5 v on the 1st Jfet drain, then connect the 2n fet drain to common (quote verbatim, and I wish I know what it exactly means, although I am Russian, I think "common" is "ground") and replace R8 with trimmer pot 6,8-10 kOhm. After that you shoud send a signal with 4-volt amplitude from generator to Q2 gate through a voltage divider 220kOhm/220kOhm. All bias circuits must be disconnected. Then turn the trimmer until you get soft clipping at maximum gain (here comes the scope  ). Then you should measure trimmers resistance and replace the trimmer with resistior of a little bit less nominal value. After that connect Q2 gate to the bias voltage source and adjust bias voltage in such a way that lower half-wave of oscillogram start clipping much earlier than upper half-wave. Bias voltage is regulated through R20 (trimmer 5,1 kOhm). The rest of cascades must be adjusted in the same way.

Hope it would help/

P.S.: OMG, I've finished!

[caspercody]

Check out this link:

http://translate.google.com/translate?u=http%3A%2F%2Fwww.sugardas.lt%2F%7Eigoramps%2Farticle68%2Farticle.htm&sl=ru&tl=en&hl=&ie=UTF-8

[MikeH]

I know this is not much, but try to find Rtotal for the source, then break it in half....if that doesn't work, find R, and make r MUCH larger than R....you may have to do some experimenting!! 

You could also use a pot set up as a voltage divider, and then measure the values for r and R after you have it set.